Why was it a point for Macha's owl's to hunt down Saoirse and Ben? And why could Macha siphon out (to an extent) Ben's feelings? Simple, Ben and Saoirse's mother was a selkie, a faerie. So that essentially makes Ben a half-faerie.
Fridge-Heartwarming: Why isn't Ben upset that his mother's seashell broke? Why isn't it ever brought up again? There's a reason why: Ben comes to recognize that the seashell isn't his mother's legacy, but Saoirse herself.
The Cameo with Aisling on the bus isn't just a cute callback to The Secret of Kells. It's Halloween, the day that the gates between the faerie realms and the human world dissolves. It makes sense to see a faerie about, and it's pure coincidence that it happens to be a faerie we recognize.
Leading to mild Fridge Horror: how many of the people in this scene are faeries we don't recognize?
The Adult Fear upon adult fear that pile up at the end when Ben and Saoirse return to the lighthouse. Conor's youngest child is sick and dying, and the only way he can get her to a hospital is to row her across a stormy sea in a tiny rowboat. En route, his son (whose very worst fear is drowning) goes overboard. Of course Conor jumps in after him, but that means leaving Saoirse alone with a dog in the rowboat—risking her dying while he searches, or the boat capsizing and drowning her too. The chances of Conor finding Ben in such a stormy sea are slim. If Conor couldn't find Ben, at some point he would have to give up and assume he was dead, while still trying to rush Saoirse to medical care. Ugh...
We as viewers know the true stakes: these kids must take the risk to save Saoirse and free the other faeries. Countless lives, including Saoirse's, depend on their actions. They are aware of the dangers and have accepted them. However, to the adults who may or may not fully believe or comprehend what's happening, all of this looks like a brother and sister playing a game that went too far and which is about to take a tragic turn. If they die, Conor won't have the consolation of knowing that they died for a good and noble cause; it's solely because he was a bad father who didn't look after them.